Biological processes depend on energy flow through the Earth system.

Energy Literacy Principle 3

Teaching about energy in biological processes is supported by 6 key concepts:

3.1 The Sun is the major source of energy for organisms and the ecosystems of which they are a part. Producers such as plants, algae, and cyanobacteria use the energy from sunlight to make organic matter from carbon dioxide and water. This establishes the beginning of energy flow through almost all food webs.

3.2 Food is a biofuel used by organisms to acquire energy for internal living processes. Food is composed of molecules that serve as fuel and building material for all organisms as energy stored in the molecules is released and used. The breakdown of food molecules enables cells to store energy in new molecules that are used to carry out the many functions of the cell and thus the organism.

3.3 Energy available to do useful work decreases as it is transferred from organism to organism. The chemical elements that make up the molecules of living things are passed through food chains and are combined and recombined in different ways. At each level in a food chain, some energy is stored in newly made chemical structures, but most is dissipated into the environment. Continual input of energy, mostly from sunlight, keeps the process going.

3.4 Energy flows through food webs in one direction, from producers to consumers and decomposers. An organism that eats lower on a food chain is more energy efficient than one eating higher on a food chain. Eating producers is the lowest, and thus most energy efficient, level at which an animal can eat.

3.5 Ecosystems are affected by changes in the availability of energy and matter. The amount and kind of energy and matter available constrains the distribution and abundance of organisms in an ecosystem and the ability of the ecosystem to recycle materials.

3.6 Humans are part of Earth's ecosystems and influence energy flow through these systems. Humans are modifying the energy balance of Earth's ecosystems at an increasing rate. The changes happen, for example, as a result of changes in agricultural and food processing technology, consumer habits, and human population size.

Energy from the Sun fuels life on Earth

The continual input of energy, mostly from sunlight, sustains the process of life. Sunlight allows plants, algae and cyanobacteria to use photosynthesis to convert carbon dioxide and water into organic compounds like carbohydrates. This process is the fundamental source of organic material in the biosphere. There are a few exceptions to this, such as ecosystems living around hydrothermal vents on the ocean floor, which derive their energy from the chemical compounds such as methane and hydrogen sulfide. In either case, the overall productivity of an ecosystem is controlled by the total energy available.

Energy flows through all life on Earth

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Diagram of a food chain for waterbirds of the Chesapeake Bay. Image from US Geological Survey.

Provenance: Image from US Geological SurveyReuse: This item is in the public domain and maybe reused freely without restriction.

Food webs show how energy moves throughout a system. Plants use energy from the Sun to create organic matter. Plants are then eaten by primary consumers who are in turn eaten by secondary consumers, and so on. In each step, the energy that was originally emitted by the Sun is consumed, but that energy also dissipates with each step. Animals use up 90% of the energy contained in the foods they eat for their normal activities. This leaves just 10% of the original energy available for the next consumer. The efficiency of the food chain decreases as you go upward. (Learn more about the transfer of energy in the food chain.)

This points out a critical factor in the distribution of energy in human foods too. Eating producers (plants) at the bottom of the food chain is the most efficient way for humans to acquire energy for living. This has implications for humans as we strive to keep a growing human population adequately nourished.

These ideas also introduce the origin of organic matter that later can become fossil fuels. The original source of energy in fossil fuels is sunlight, which fueled photosynthesis. Oil and natural gas come from photosynthetic plankton, which have been preserved in sediments on the ocean floor, heated, and chemically altered into hydrocarbons. Coal comes from plants that have been buried in sediment, compacted, and preserved. These ideas are further explored in Energy Principle 4. (Learn more about where oil comes from.)

Helping students understand these ideas

While many students can readily relate to the idea of the terrestrial food web, the marine food web may be less familiar to them. Students may be surprised to learn that about half the Earth's primary productivity of organic material comes from the oceans.

Other topics that tie into this theme are:

examining methods of measuring primary productivity in different ecosystems,

mapping the distribution of primary productivity across the oceans and on land,

calculating the available energy in different trophic levels,

calculating the embodied energy in different foods,

considering the science, technology, economics or ethics of agriculture and livestock production,

examining various impacts to the energy balance of ecosystems, such as fires, disease, population dynamics, and changes in land use.

Bringing these ideas into your classroom

A variety of foods, each with its own embodied energy and environmental footprint.

Provenance: Image from Microsoft image galleries.Reuse: If you wish to use this item outside this site in ways that exceed fair use (see http://fairuse.stanford.edu/) you must seek permission from its creator.

Compared to Energy Principles 1 and 2, this principle is more concrete and easier to visualize. We all have direct experiences with different types of foods. Many of these concepts, such as how sunlight drives photosynthesis and food webs, are commonly taught in middle school and high school curricula. Educators can take these opportunities to tie in an energy theme with these topics.

A quantitative approach can be used to examine the energy embodied in different types of foods. Here are some examples of activities that do this.

How Much Energy is on my Plate? leads students through a sequence of learning steps that highlight the embedded energy that is necessary to produce various types of food. leads students through a sequence of activities that highlight the embodied energy that is necessary to produce various types of food (high school or introductory-level college).

The Lifestyle Project challenges students to dramatically lower their energy use, and adopting a vegetarian diet is one of the paths that students may elect to take. This project can be used with middle school through college audiences.

Related teaching materials

A hands-on way to teach these topics is from the point of view of a meal or a community garden. All of the concepts contained in this principle can be illustrated in a garden that produces food.

Teaching materials from the CLEAN collection

Middle school

To Boldly Go... is a web-based activity tackles the broad reasons for undertaking ocean exploration - studying the interconnected issues of climate change, ocean health, energy and human health. Students examine the types of technology ocean scientists use to collect important data.

The video The Ocean's Green Machines further explores the marine food web by examining phytoplankton. These organisms form the base of the marine food web, are the source of half of the oxygen on Earth, and are an important means to remove CO2 from the atmosphere. This video is suitable for a middle school or high school audience.

Stressed Out! is an activity where students research various topics about ocean health, such as overfishing, habitat destruction, invasive species, climate change, pollution, and ocean acidification. An optional extension activity has instructions to create an aquatic biosphere in a bottle and then manipulate variables.

Food webs are commonly taught with concept maps, such as with the Oceanic Food Web. The concept map-like connections on this visualization encourage students to link the abiotic and biotic interactions within the oceanic food web. This is also suitable for introductory college students.

How Much Energy is on My Plate? leads students through a sequence of learning steps that highlight the embedded energy that is necessary to produce various types of food. Students start by thinking through the components of a basic meal and are later asked to review the necessary energy to produce different types of protein.This activity may be paired with The Lifestyle Project.